RC Calibration is off for some people?

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From dasaki at https://forum.openwrt.org/viewtopic.php?pid=319315#p319315 :

I had problems detecting the an tiny RTC (DS1307) via the tiny-usb-i2c/LitteWire. After some trials it worked when I changed the clock calibration settings in main.c:

removing the "0.5" from line:

int         x, optimumDev, targetValue = (unsigned)(1499 * (double)F_CPU / 10.5e6 + 0.5);

to:

int         x, optimumDev, targetValue = (unsigned)(1499 * (double)F_CPU / 10.5e6 );

Hope it helps someone else.

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It looks like there has been a bunch of work on this since I last poked at it.

It is claimed FunkUSB (https://www-user.tu-chemnitz.de/~heha/viewzip.cgi/basteln/PC/FunkUsb/FunkUsb.zip/FunkUsb/com/ ) continually recalibrates through some kind of timer method.

micronucleus/micronucleus#26 goes through a lot of detail on how calibration currently works on ATtiny85, and @embedded-creations says “...also surprised the default routine works at all.”

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See https://github.com/micronucleus/micronucleus/blob/master/firmware/osccalASM.S